Photoresist is typically employed as block mask during ion implantation for front-end-of-line (FEOL) CMOS device processing. The block mask of the photoresist is used to selectively allow ions being implanted across an entire wafer to penetrate into desired regions of the substrate. Once the implantation process is performed, the photoresist is removed from the wafer.
Removal of resists exposed to ions during this processing is currently carried out by an oxygen plasma ash method followed by a wet stripping (or cleaning) process. The follow-up wet stripping is performed to remove resist particulate that remains on the wafer after the dry stripping (oxygen plasma asking). In addition to causing particulate contamination, the dry stripping methods using oxygen plasma may also cause silicon loss. Current wet cleaning methods based on high temperature (>180° C.) sulfuric acid-peroxide mixtures (SPM) claim to remedy the particulate contamination and silicon loss of the dry stripping method. However, the high temperature and extreme low pH conditions used in SPM based resist stripping pose a serious safety concern.
In addition these high temperature SPM wet cleaning methods have varying degrees of effectiveness in removing carbonized crusts formed in resists implanted at dosages ≧1016/cm2. In particular, high dose implants damage the upper layer of a photoresist, forming a cross-linked (carbonized) crust layer. Not only is the crust layer very difficult to remove using wet chemical based processes, but removing the crust by undercutting of the undamaged or lightly damaged regions of photoresist can result in defects on the wafer due to re-deposition of fragments from the crust. In addition, the part of the crust in contact with the silicon near the edge bead removal region (EBR) remains unaffected in many wet chemical systems.
Very hot (>200° C.) SPM, generated at the wafer level, has shown some promise in attacking the crust and stripping the resist; however, hot SPM is not compatible with metal gates used in a number of current devices.
Thus, there is a continued need in the art for effective photoresist removal.